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Abstract:

A thermocline storage tank is presented, which includes a barrier member
that floats between the two fluids stored at different temperatures,
physically separating and insulating them. The floating barrier includes
a number of design features that broaden its application scope, enabling
it for use in fields like thermal storage systems of solar power plants.

Claims:

1. Dual thermal energy storage tank comprising a barrier which floats in
the interface of two masses of fluid stored at different temperatures,
due to the difference in densities between said masses of fluid, and
where said barrier has an intermediate density between those of the
stored fluids at its different nominal temperatures wherein said barrier
comprises, at least, a fluid-tight outer shell and a filler material
inside the fluid-tight outer shell; and in that said filler material is
made of rigid and compression resistant material, and laid inside the
outer shell in loose form, without providing any restriction to the
thermal growth between the different elements.

2. Dual thermal energy storage tank of claim 1 wherein the second filler
material further comprises, at least: a first horizontal insulating
layer; a second horizontal density adjustment layer.

3. Dual thermal energy storage tank of claim 1 wherein the second filler
material further comprises a separating layer between the first and
second horizontal layers; said means in such way that said first and
second layer are kept separated in order to prevent any potential mixing
between the layers.

4. Dual thermal energy storage tank of claim 1 wherein the fluid-tight
outer shell is made in the same construction material of the tank shell
and in that said material is carbon steel for upper operating
temperatures below 400.degree. C.-450.degree. C. and stainless steel for
upper operating values above 400.degree. C.-450.degree. C.

5. Dual thermal energy storage tank of claim 1, wherein the materials of
the first and second horizontal layers are supplied in granular form or
in small single pieces.

6. Dual thermal energy storage tank of claim 1 wherein the barrier
further comprises a plurality of external ballasts in order to provide
additional weight adjustments or to balance the barrier once the barrier
is finished and fully closed.

8. Dual thermal energy storage tank of claim 1 wherein the fluid-tight
outer shell of the barrier consists of a single body comprising: a first
upper plate; a second bottom plate; a third vertical plate closing the
peripheral space between the first and second plates

9. Dual thermal energy storage tank of claim 8 wherein at least one of
said first upper plate and second bottom plate have non-planar geometry.

11. Dual thermal energy storage tank of claim 8 wherein the third
vertical plate has a waved or corrugated shape for the circumferential
cross-sectional contour line of the plate.

12. Dual thermal energy storage tank of claim 1 wherein the
circumferential cross-sectional contour line of the barrier outer shell
has a number of waved or straight lobes near its outer perimeter, in
order to increase the flexibility in the connection between the upper and
lower plates of the barrier shell and consequently reduce the thermal
deformation of the barrier shell.

13. Dual thermal energy storage tank of claim 1 wherein said barrier is
divided into a plurality of separate and independent bodies, each of the
bodies comprising: a fluid-tight outer shell; and a filler material
inside the fluid-tight outer shell; and in that said filler material is
made of rigid and compression resistant materials, and laid inside the
outer shell in loose form, without providing any restriction to the
thermal growth between the different elements.

14. Dual thermal energy storage tank of claim 13 wherein the different
bodies of the barrier are assembled to each other, in such way that their
cohesion is assured while relative freedom is allowed between them, due
to the strings or chains that assemble adjacent bodies.

16. Dual thermal energy storage tank of claim 15 wherein at least one
closing collar for the barrier passing holes are provided in the form of
expansion joints or flexible metallic hoses, in order to have enough
flexibility to adequately accommodate the difference in thermal
expansions between the upper and lower plates of the barrier shell.

17. Dual thermal energy storage tank of claim 15 wherein at least one
hole is engaged to one column fixed to the tank,

18. Dual thermal energy storage tank of claim 17 wherein said column has
a tubular section in order to minimize heat conduction through it and to
permit its use for other purposes, such as instrumentation pass or fluid
conduction.

19. Dual thermal energy storage tank of claim 1 wherein the barrier also
comprises a plurality of ribs attached to both the upper and lower plates
of the fluid-tight outer shell, in order to provide structural strength.

20. Dual thermal energy storage tank of claim 19 wherein said ribs have
the additional purpose of preventing any radial separation between the
filler material and the fluid-tight outer shell.

21. Dual thermal energy storage tank of claim 1 wherein a number of legs
are added to the barrier, in order to support its weight and to limit its
downward motion inside the tank.

Description:

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention generally relates to the field of thermal
energy storage systems, and more particularly to the improvements in the
design of thermocline storage tanks.

BACKGROUND OF THE INVENTION

[0002] Thermal energy storage systems are generally used in applications
where it is necessary to decouple energy collection from energy delivery.
Solar energy collection systems are a typical example of this, as there
may normally exist a demand for energy during periods without solar
radiation, when no energy can be collected, but energy has still to be
delivered to satisfy said demand.

[0003] The size of solar energy collection systems may range from small
domestic collector systems, used for heating water, to much larger
collector systems, as those encountered in solar electric power plants.

[0004] One way of storing thermal energy consists of employing the
sensible heat of a fluid. During periods with solar radiation, thermal
energy is stored by heating said fluid, so that upon cooling the fluid
during periods without solar radiation thermal energy will be delivered
to satisfy the energy demand during those periods, in which energy
collection is not available.

[0005] A common design of a sensible heat storage system in solar power
plants comprises two storage tanks, which hold a volume of thermal fluid.
Each of the tanks contains said fluid at a different temperature, so that
one of the tanks contains a volume of thermal fluid at a given "cold"
temperature, and the other tank contains a volume of thermal fluid at a
given hotter temperature.

[0006] During operation of the plant, in periods with solar radiation,
thermal fluid is withdrawn from the cold tank and is heated using with
thermal energy coming from the solar collector system, then pouring it
into the hot tank. In periods with no solar radiation, thermal fluid is
withdrawn from the hot tank, making it flow through a heat exchanger
where it is cooled, thus providing the necessary thermal energy for
electric power generation.

[0007] It must be noted that with the described storage system, each of
the tanks has to be sized to hold the entire volume of the thermal fluid,
so that the total storage capacity of the system is actually twice the
total volume of the thermal storage fluid inventory of the plant.

[0008] In practice, the tanks of the storage system of a solar power plant
can reach considerable sizes, and the need for the aforementioned
"redundant" storage volume leads to several drawbacks in terms of
fabrication costs of the additional tank, increased thermal losses of the
storage system or the costs of the auxiliary equipment, piping, etc.,
associated with the additional tank.

[0009] So, it becomes desirable to eliminate the redundant volume from the
storage system, and there are currently several approaches offering
solutions to this problem. The most common solution is the thermocline
tank, in which the entire volume of the thermal fluid is hold in a single
tank. In this single tank, the two masses of cold and hot fluid are
stored one atop the other, and the natural stratification or thermocline
resulting from the difference in densities of the fluid at the two
different temperatures keeps them substantially separated. That is, the
cold fluid, which is normally denser than the hot fluid is stored below
the hot fluid, and the buoyant forces resulting from this difference in
densities helps to maintain the two masses of fluid separated, with a
rather steep temperature change in the interface between them.

[0010] When thermal energy is being collected, cold fluid is extracted
from the bottom of the tank and heated fluid is returned to the top of
the tank, and when thermal energy has to be delivered hot fluid is
retrieved from the top of the tank, and cold fluid is returned to the
bottom of the tank.

[0011] As the quantity of the fluid at one of the temperatures being
extracted from the tank is always essentially equal to the quantity of
fluid that is introduced at the other temperature, the total mass of the
stored fluid in the tank remains essentially constant through the whole
operation cycle of the storage system. In this way, the single
thermocline tank is always working at its full capacity (i.e., is full,
or nearly full, of stored fluid), optimizing the storage efficiency.

[0012] However, several phenomena like the conductive heat transmission
between the two masses of fluid, or the convective currents resulting
from the combined effect of natural stratification and edge energy losses
of the tank can significantly degrade the vertical thermal profile of the
fluid contained in the tank, particularly when the interface region is
near the bottom or the top of the tank.

[0013] A variant of the described thermocline tank is the mixed-media
thermocline tank, in which the tank is filled not only with the thermal
fluid, but also with some kind of solid material. The solid material
contributes to the total thermal capacitance of the system, and is
normally cheaper than the thermal fluid. Besides it helps inhibiting
convective mass transfer between the cold and hot fluids, making the
thermocline more effective than in the case of a single media storage
tank.

[0014] However, some issues arise related to the use of a mixed
fluid-solid storage media, and these include:

[0015] (a) The compatibility and long-term physical/chemical stability of
the solid media in contact with the thermal fluid and subjected to
thermal cycling.

[0016] (b) The settlement of the solid media on the bottom of the tank as
a result of the repeated cycles of operation, resulting in increased
stresses in the tank walls near the bottom, and leading to the need of
thicker tank walls.

[0017] Several patents have already described thermocline storage concepts
similar to these, e.g., U.S. Pats. No. 4,124,061 and 5,197,513.

[0018] In the present patent application an improved variant of the
thermocline storage system is described. In the described solution, a
horizontal physical barrier is employed to separate and thermally
insulate the two masses of fluid. The physical barrier has an
intermediate density between the higher density of the cold fluid and the
lower density of the hot fluid, so that it floats in the interface
between the two fluids and it travels together with this interface in a
vertical direction inside the tank.

[0019] Due to this feature, the barrier member travels vertically inside
the tank following the interface between the stored hot and cold fluids,
naturally achieving a vertical position coincident with that of said
interface.

[0020] Considering as an example the typical daily working cycle of the
storage system of a solar thermal power plant, at the first hour in the
morning the single storage tank considered in this invention is full of
colder fluid, maybe with just a minimum heel of hotter fluid left on the
top, and the barrier member is near the top of the tank.

[0021] During the day, as thermal energy is collected from the solar
field, colder fluid is extracted from the tank, at the same time that
hotter fluid is introduced into the tank. As the quantity of hotter fluid
in the tank increases and the quantity of colder fluid decreases, the
interface region between the hotter and colder fluids moves vertically
towards the bottom of the tank, with the barrier member following it. In
this way, at some point during the thermal energy collection period from
the solar field, the storage tank is full of hotter fluid, maybe with a
minimum heel of colder fluid left at the bottom of the tank, and the
physical barrier is near the bottom of the tank.

[0022] The trip of the barrier from its highest position in the tank to
its lowest position takes place in the charging period of the tank. The
discharging period, which completes the whole typical daily cycle of the
tank, occurs in a similar way, with hotter fluid being extracted from the
top of the tank and colder fluid being introduced to the bottom of the
tank, and with the barrier moving vertically from the bottom of the tank
up to the upper part of the tank.

[0023] The use of a physical barrier between the two masses of fluid
prevents the mass transfer between the two regions and greatly reduces
conductive heat transmission between them, thus significantly improving
the performance of the thermocline. At the same time, it avoids the
disadvantages related to the use of a mixed-media storage solution.

[0024] The general arrangement of the physical barrier consists of an
outer, fluid tight shell, and an insulating material, which is placed
inside the mentioned shell. The concept of the physical barrier
separating the two masses of fluid was already described in U.S. Pat. No.
4,523,629. In said patent, a particular embodiment of the barrier,
suitable for application in the storage of water between 100° F.
and 175° F., is illustrated. The patent also mentions the possible
application of the invention in solar power plant storage systems, but no
specific configuration for this application is disclosed.

[0025] However, it must be noted that there are several problems that
affect the physical barrier, and that must be tackled in order to produce
a feasible and reliable design of it. These problems are not critical in
the conditions of the water storage application described in the
aforementioned patent, but become more severe in the more demanding
conditions seen in solar power plant energy storage systems, with higher
temperatures and temperature differences between the stored fluids.

[0026] From the explanation of these problems, it will be understood that
a need exists for specific solutions in order to solve or at least
alleviate these problems. What this patent intends is precisely to put
forth these solutions, which will greatly improve the features of the
invention and extend the application scope of it.

[0027] In order to make more apparent the problems the physical barrier
has to face, some operating conditions for the storage tank, typically
seen in real solar power plant storage systems, will be considered as an
example.

[0028] The particular case considered is the storage of a mixture of
molten nitrate salts between 292° C. and 386° C., in
cylindrical vertical tanks of approximately 15 m in height and 40 m in
diameter.

[0029] One of the problems affecting the physical barrier belongs to its
possible construction materials. The barrier member described in U.S.
Pat. No. 4,523,629 consists of a fluid-tight shell, made of some plastics
like polycarbonate and Plexiglas®, and some insulating material, like
urethane foam or fiberglass, encapsulated into this shell. As stated in
that patent, the functions of the shell in the described barrier member
are to prevent water absorption and to provide structural stiffness to
maintain the predetermined configuration of the barrier member.

[0030] However, temperature ranges typically present in solar power plant
storage systems, are well above the allowable limits for plastics, so
another kind of materials have to be considered for the construction of
the barrier.

[0031] In addition, for the static pressure values of common solar power
plant storage tanks, and considering the huge size of the barrier member
necessary for these tanks, it is virtually impossible that the outer
shell alone could stand this pressure load, maintaining a nearly constant
volume.

[0032] As mentioned in U.S. Pat. No. 4,523,629 one constraint the physical
barrier needs to fulfill is related to its density: in order to float in
the interface of the hot and cold fluids, an adequate combination of
materials must be selected for the construction of the barrier, so that
an intermediate density between the ones of the hot and cold fluids is
achieved. Besides, it is necessary for the barrier to have enough
structural strength in order to maintain its volume nearly constant under
the full range of static load imposed by the stored fluid.

[0033] In said patent, the described way of adjusting the weight of the
barrier member consists of adding some exterior weights, so that the
desired density is achieved. While this could be an adequate solution for
small barriers, in the case of big barriers, it is likely that the
necessary weights would be excessively big, becoming quite a
non-efficient solution at least for gross-weight adjustments.

[0034] Another problem affecting the physical barrier is related to its
thermal deformations in service. As a result of having the upper surface
of the barrier at the hot temperature and the lower surface at the cold
temperature of the stored fluid, an overall state of bending deformation
is developed in the barrier, in order to accommodate the differential in
thermal expansion between the upper and lower parts of it. For example, a
plane disk, made of common carbon steel, 30 cm thick, with a diameter of
40 m and with a temperature difference across its thickness of 94°
C., will adopt a spherical deformed shape, and its maximum deflection
will be in the order of 0.9 m.

[0035] These big deformations decrease the useful height of the storage
tank, and in addition, they could lead to structural problems in the
barrier. Apart from this, a curved barrier member alters the naturally
plane interface between the two stored fluids, and thus, it is likely
that fluid will pass from one side to the other of the barrier, so that
the natural plane shape of the interface is restored. In this way, much
of the insulating capacity of the barrier member is lost.

[0036] Economic factors have to be regarded as well in the design of the
barrier member, in order to produce a cost effective design. It has to be
taken into account that if a single tank with a barrier is more expensive
than two tanks, the traditional two-tank option would always be
preferable. This means that the choice of materials for the barrier, as
well as the fabrication method, is very important in the barrier design.
Therefore, it is very important for the materials considered for the
barrier to be cheap and widely available.

[0037] The barrier member described in this patent application includes
design features that provide solutions to all these mentioned problems,
and which are outlined throughout the text.

[0038] The description of the invention is merely exemplary in nature and,
thus, variations that do not depart from the gist of the invention are
intended to be within the scope of the invention. Such variations are not
to be regarded as a departure from the spirit and scope of the invention.

SUMMARY OF THE INVENTION

[0039] The present invention relates to thermal energy storage tanks, and
more specifically to a thermocline storage tank which includes a barrier
member that physically separates the two masses of fluid stored at
different temperatures.

[0040] The barrier member object of the present invention overcomes the
formerly mentioned problems, due to a number of design features that
enhance its use and extend its applicability to fields and application
areas for which no specific design solutions or configurations have been
provided so far, e.g. thermal storage in solar power plants.

[0041] The storage tank considered for the invention is preferably of the
vertical cylindrical type, although any other type of tank can be
considered as well within the scope of application of the invention, as
long as it has an essentially uniform horizontal or cross section along
its entire height or longitudinal axis (i.e., it is of prismatic shape),
so that the floating barrier member can freely travel inside the tank
along its longitudinal axis.

[0042] The barrier member essentially consists of one fluid-tight outer
shell, and some filling material(s) which are placed inside the shell.
The barrier member has an intermediate density between those of the
stored fluid at its different nominal temperatures, so that it floats in
the interface between the two masses of stored fluid.

[0043] The cross section of the barrier member is preferably of the same
shape as the cross section of the tank, so that it effectively covers the
contact area between the fluids stored in the tank at different
temperatures, and is able at the same time to freely travel along the
longitudinal axis of the tank. Thus, in the case of a cylindrical
vertical tank, the barrier member would have the form of a disk, of
approximately the same diameter as the tank, and with enough thickness to
adequately separate and insulate the two masses of stored fluid.

[0044] Nevertheless some clearances or gaps between the barrier outer
border and the tank shell can be left, in order to account for tolerances
or different possible deviations from theoretical shape in manufacturing,
or expansions and deformations in service, for example.

[0045] Additionally, a number of longitudinal passing holes can be
performed in the barrier, with the purpose of serving for piping or
instrumentation pass, guiding, etc.

[0046] The novel features of the invention, which make it suitable for use
in applications such as thermal energy storage in solar power plants,
include:

[0047] (a) Providing loose and compression resistant materials as the
filler materials for the barrier, which eliminates any problems related
to thermal deformations in the filler material and enables the barrier to
easily withstand the pressure load of the stored fluid and maintain a
nearly constant volume without having to add a complex and costly
structure to its outer shell.

[0048] (b) Dividing the interior filler material of the barrier into two
layers, one of which is an insulation layer and the other a weight
adjustment layer, achieving in this way an effective manner of easily
adjusting the density of the barrier to the desired value.

[0049] (c) Providing the outer shell of the barrier with non planar
geometry in one or both of its upper and lower faces, which greatly
increases its stiffness and reduces its thermal deformations.

[0050] (d) Adding waved or straight circumferential lobes on the outer
zone of the barrier shell, so that the connection between the upper and
lower faces of the shell is made much more flexible and the thermal
deformations and stresses are substantially reduced.

[0051] (e) Breaking out the barrier member into a plurality of smaller and
independent bodies, arrayed one aside the other to complete a modular
barrier, which reduces to a great extent the problems related to thermal
deformations, as well as the manufacturing problems, present in a single
bigger component.

[0052] The use and applicability of the present invention, including these
and other novel features, will become more fully understood from the
detailed description provided hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Following it is briefly described some figures that help to better
understand the invention. The figures also describe an embodiment of the
present invention, as non-limitative example:

[0054]FIG. 1 is a schematic vertical cross-sectional view of the dual
thermal energy storage tank considered in this invention, showing the
general arrangement of the two masses of fluid and the barrier member
inside the tank.

[0055] FIG. 2 is a vertical cross-sectional view of the barrier object of
the present invention, showing several details of it in a first preferred
embodiment.

[0056] FIG. 3a shows a horizontal cross-sectional view of the barrier,
taken along line 3-3 of FIG. 2.

[0057] FIG. 3b is a schematic top view of the barrier, showing only an
exemplary arrangement of a number of holes in it.

[0058] FIG. 4a represents a vertical view of one half of the barrier
member in a second preferred embodiment, with a partial section showing
the interior structure and filler material.

[0059] FIG. 4b is a partial vertical view of the outer shell of the
barrier, showing an alternative configuration for the contour line of the
outer zone of this shell to that represented in FIG. 4a.

[0060] FIG. 5 is a top view of the barrier member in a third preferred
embodiment, showing an exemplary break out of it.

[0061] FIG. 6 is an enlarged view representing an exemplary connection
between the different bodies of the barrier shown in FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0062]FIG. 1 shows the schematic arrangement of a thermal storage system
(1), which can be the storage system of a solar thermal power plant. The
storage system (1) includes a thermocline storage tank (2), which stores
two masses of fluid at different temperatures. The mass of colder fluid
(4) is normally denser than the mass of hotter fluid (3), and is stored
below it. The tank can typically be of the vertical cylindrical type,
with a diameter of about 40 m and a height of about 15 m. In many common
solar applications, the cold fluid will usually be at a temperature of
about 300° C., and the hot fluid will be at a temperature of about
400° C., and the fluid stored at both temperatures will typically
be a mixture of molten nitrate salts.

[0063] The barrier member object of the present invention, represented
schematically in in FIG. 1 and designated by numeral (13), is located in
the interface between the hot and cold fluids, physically separating and
insulating them, so that the heat conduction between the two masses of
fluid is minimized.

[0064] As stated previously, the barrier member essentially consists of an
outer fluid tight shell, this shell being essentially of the same shape
as the cross section of the tank, and some filling material(s) that are
put inside this shell filling its interior space.

[0065] The outer shell of the barrier is preferably manufactured in the
same material as the tank shell, which would likely be carbon steel for
upper operating temperatures below 400-450° C., and stainless
steel for upper operating temperatures above this value.

[0066] For the particular case being considered, the average thickness of
the barrier will be preferably in the order of 0.2-0.4 m in all of the
proposed embodiments, so that and adequate insulation between the fluids
is achieved, without occupying an excessive space inside the tank.

[0067]FIG. 1 also outlines how thermal energy is collected or extracted
from the tank. When thermal energy is being collected, cold fluid is
extracted from the bottom of the tank via the cold fluid exit line (5),
by means of a cold pump (6). The fluid is circulated through a heat input
device (7) where it is heated, returning then to the top of the tank via
the hot fluid inlet line (8). On the other hand, when thermal energy is
being extracted, hot fluid is extracted from the top of the tank via the
hot fluid exit line (9), by means of a hot pump (10), which forces it
through a heat extraction device (11) where it is cooled, returning then
back to the tank via the cold fluid inlet line (12).

[0068] The necessary measuring devices can be added both to the barrier
member and to the thermocline tank, in order to properly monitor and
control the operation of the storage system. The instrumentation of the
system can include, for example, an array of vertically disposed
thermocouples to obtain the vertical temperature distribution of the
tank, and level transmitters, to monitor the total height of the stored
fluids, the vertical position of the barrier inside the tank, and the
horizontality of the barrier.

[0069] Even though the heat input device (7) and the heat extraction
device (11) are represented as separate components in FIG. 1, in
commercial solar power plants they will usually be the same single
device, likely an oil-to-molten salt heat exchanger.

[0070] Referring to FIG. 2, the fluid tight outer shell of the barrier
(21) essentially comprises a top plate (21a), a bottom plate (21b) and a
peripheral vertical closing plate (21c) connecting the top and bottom
plates.

[0071] In normal operation, a vertical temperature gradient will be
developed across the thickness of the barrier, and the temperatures of
the bottom and top plates of the barrier shell will essentially be those
of the stored cold and hot fluids respectively. As a result of this
temperature distribution, there will be a differential between the
thermal expansions of the upper and lower parts of the barrier, and a
state of thermal stresses and deformations will be developed in the
barrier shell.

[0072] While the problem of differential thermal expansion in the filler
material is solved due to its granular or small-brick form as will be
explained later, this problem still remains for the outer shell of the
barrier. Some design features are provided for the barrier in order to
solve this problem, which are introduced in different embodiments
proposed for the barrier.

[0073] In a first embodiment, as can be seen in FIG. 2, the top plate
(21a) of the barrier is given a non-planar shape, like for example a
conical or a spherical shape (in this case a conical shape is
represented). Due to this feature, the stiffness of the barrier shell is
greatly increased, and consequently the overall bending of the entire
barrier due to the thermal gradient across it is radically reduced.

[0074] Even though the tapering of the upper plate (21a) is represented in
a pronounced manner in FIG. 2, in practice the necessary tapering will be
much less pronounced, and the maximum separation between the upper and
lower plates, achieved on the outer border of the barrier, will
preferably be on the order of 0.5 m.

[0075] Another problem in the outer shell of the barrier are the high
stresses present in the vertical closing plate of the shell, as a result
of the difference between the upper and lower plate radial expansions it
has to accommodate.

[0076] This problem is solved in two ways; firstly increasing the vertical
distance between both plates in the perimeter, and secondly reducing as
much as possible the thickness of the vertical plate (21c), so that the
flexibility of this vertical plate is increased. The thickness reduction
of the vertical plate has the additional advantage of reducing the heat
conduction going through this plate from the hot side to the cold side of
the tank.

[0077] FIG. 2 also depicts the different filler material layers for the
barrier, referenced by (22) and (23). As seen in the Figure, the filler
material inside the barrier is preferably separated in two different
horizontal layers. One of the layers (23) serves for insulation purposes,
i.e., gives the barrier its insulating capacity, and being normally
lighter than the other layer, is preferably located atop the second
layer. The second layer (22) is the density adjustment layer, and its
purpose is to adjust the total weight of the barrier so that the final
desired density is achieved. Between both layers, a metal foil (24) can
be added, so that both filler layers are kept physically separated and
any potential mixing between the materials of both layers is prevented.

[0078] The materials of both layers have the additional feature of being
rigid and compression resistant. In this way, the filler material of the
barrier is basically the responsible of withstanding the pressure load of
the stored fluid and maintaining a nearly constant volume of the barrier.
In this way, the heavy and expensive structure that the outer shell of
the barrier would need, if filled with "soft" materials, is avoided.

[0079] Furthermore, in order to eliminate the problems related to thermal
deformations in the filler materials, the materials of both layers are
supplied in granular form or in small single pieces, like bricks for
example, and in the construction of the barrier, the filler materials are
laid inside the outer shell in loose form, without providing any
restriction to the thermal growth between the different pieces. In this
way, the problems related to differential thermal expansion that a single
big monolithic component would have are avoided, and, additionally, the
filling materials can flow in the space inside the barrier, so that all
the interior spaces and voids are conveniently filled.

[0080] Several kinds of refractory bricks, as well as different types of
expanded clay in granular form such as perlite, vermiculite, or arlite;
as long as an adequate packing or ramming of the bulk filling material is
guaranteed so that no settlement and therefore no significant volume
changes occur during operation of the barrier, are believed to be
suitable materials for the insulating layer of the barrier. These
materials have a low thermal conductivity, adequate stiffness and
compression resistance and can operate at temperatures higher than those
typically present in solar power plant storage tanks. Besides, they are
quite common materials used in construction, and have a reasonably low
price.

[0081] As for the material of the other layer of the barrier, its most
important physical feature, apart from its stiffness and compression
resistance, is its density. Sand, cement, and various types of rock can
be suitable materials for this layer. Even though it would be desirable
to have a single insulating material as the filler for the barrier, it
may be that no suitable material which fulfils both the adequate density
and low thermal conductivity requirements is available.

[0082] Considering, for example, a typical case in which the stored fluid
is a mixture of molten nitrate salts between the temperatures of about
300° C. and 400° C., with densities at the cold and hot
temperatures near 1840 and 1900 kg/s respectively, the required density
for the filler material of the barrier can very well be in the range of
1000 kg/m3 or higher.

[0083] The suitable insulating materials proposed above, however, have
density values quite below this range, and it is foreseen that a suitable
design of the barrier for a common molten salt storage tank in a solar
power plant will have too little weight, if only filled with any of those
insulating materials.

[0084] In order to solve this situation the filler inside the barrier is
divided into two layers, as explained previously. One of the layers has
the responsibility of providing its insulating capacity to the barrier,
and the other layer provides the necessary gross weight adjustment, so
that the desired density for the barrier is achieved.

[0085] Additional final weight adjustments may be made to the barrier once
it is finished and fully closed, by attaching a number of exterior
ballasts to it. These exterior ballasts can be both rigidly attached to
the barrier member, or simply laid on it, so that weight can be added or
removed from the barrier once it is in operation, to further adjust its
weight and density. This can be accomplished, for example, by means of a
number of weights, that are placed on the top of the barrier and that can
be removed at any time from the top of the tank, in order to replace them
with heavier or lighter weights.

[0086] These exterior ballasts are represented in FIG. 4a, referenced by
numerals (33) and (34). As can be seen in this Figure, ballasts (33) are
permanently fixed to the outer shell of the barrier, either to its bottom
or to its top plate. Welding is the preferred method of attaching these
ballasts to the barrier shell. On the other hand, adjustable ballasts
(34) are simply laid on the top face of the barrier, and can be removed
and replaced by other lighter or heavier weights at any time, by means of
strings (35), which go up to the tank roof and out of the tank through
some holes performed in the tank roof. The adjustable ballasts (34) can
also be used to properly balance the barrier, if necessary.

[0087] Referring again to FIG. 2, some passing holes (26) are preferably
added to the barrier. Some vertical closing collars (28) are added for
each of the holes, welded to both the top and the bottom plate. These
holes can serve for guiding the movement of the barrier inside the tank,
which can be accomplished by means of vertical columns (27) engaged into
these holes and fixed to the tank.

[0088] For the vertical closing collars (28) of the barrier holes (26), it
has to be taken into account that they have to accommodate a differential
in radial thermal expansion between the upper (21a) and lower (21b)
plates of the barrier outer shell (21). For this reason, they are
preferably provided in the form of expansion joints or flexible metallic
hoses, with a waved contour line (not shown in the Figure) that provide
them with enough flexibility to accommodate said differential in thermal
expansion between the upper (21a) and lower (21b) plates of the barrier
outer shell (21).

[0089] Columns (27) are preferably of tubular section, in order to
minimize the heat flux going through these columns from the hot side to
the cold side of the tank. Holes (26) can have other additional
functions, such as serving for instrumentation, pipelines, etc. passage.
FIG. 3b is a top view of the barrier shell, showing only an exemplary
arrangement of some holes in the shell. As can be seen in this Figure,
holes which are offset from the central axis of the barrier are elongated
in the radial direction of the barrier, in order to accommodate its
radial expansions.

[0090] Some structures of ribs (29), made with standard extruded profiles,
are added to both the upper and lower plates of the barrier shell (21).
The ribs for the lower plate provide this plate with enough structural
strength to withstand the own weight of the barrier before it enters in
service. This structure is preferably located above the lower plate
(21b), thus inside the barrier shell, having the additional function of
dividing the interior space of the shell into separate compartments with
the purpose of a better guiding for the placement of the filling
materials inside the shell. On the other hand the ribs for the upper
plate (21a) increase the stiffness of this plate so that buckling of the
plate is avoided.

[0091] Additionally, the rib structures of the upper and lower plates have
the function of keeping the filler material in the peripheral region of
the barrier in close contact with the vertical closing plate, preventing
any separation between the filler material and the vertical closing plate
that could come as a result of differences between the radial thermal
expansions of the outer shell of the barrier and the inner filler
material.

[0092] In order to adequately support the barrier before it enters in
service, and also in order to limit its downward motion inside the tank
once in service, a number of legs, represented schematically by (25), are
fixed below the lower plate (21b) of the barrier. FIG. 3a shows an
example of a possible arrangement of the ribs (29) and of the fixed legs
(25) in the bottom plate (21b).

[0093] Yet another way of further improving the performance of the outer
shell with respect to thermal deformations is presented in FIG. 4b, where
a second preferred embodiment for the invention is depicted. As seen in
this Figure, some circumferential waved lobes (32b) are implemented in
the peripheral region of the barrier. This feature adds flexibility to
the coupling between the upper and lower plates of the shell, so that
they are partially decoupled from each other. In this way, the connection
between the upper and lower plates (21a, 21b) behaves like a flexible
joint, thus enabling each of the plates to freely achieve their
corresponding expanded dimensions.

[0094] In order to make the manufacturing easier said circumferential
lobes can be made out of straight sections, like the ones shown in FIG.
4a, referred to as (32a). This Figure also includes a partial section
which shows an exemplary arrangement of the filler material inside the
barrier as an array of bricks (36) (no distinction between the different
layers of the filler material is made in this Figure).

[0095] In another configuration of the invention, schematically outlined
in FIG. 5, the barrier is divided into a number of separate and
independent bodies (51), each of the bodies having its own fluid-tight
metal outer shell with its corresponding filling material layers inside.
As an example, one way of dividing the barrier could be breaking it into
one circular central piece and a number of outer annulus sectors.

[0096] The advantages of this configuration come from the fact that the
size of each of the independent bodies is reduced, thus considerably
reducing the problems related to differential thermal expansions in the
barrier. Besides, the construction of the barrier is enhanced due to the
modularity of this configuration.

[0097] In order to avoid any vertical separation of the different bodies,
they are assembled to each other in such way that their cohesion is
assured, while some relative freedom is permitted between them, so that
each body behaves as an independent piece. This can be accomplished by
providing a number of lugs (52) to the outer edges of each body, so that
adjacent edges of adjacent bodies can be tied to each other by means of
strings or chains (53), or other means of the like.

[0098] In the proposed configurations for the barrier shell, a high heat
flux is conducted through the vertical closing metal plate (21c), which
has a high thermal conductivity and thermally connect both zones of the
tank at different temperatures.

[0099] One additional feature can be introduced in the barrier, which
seeks to reduce the heat flux going through the vertical plate (21c).
This feature consists of giving a curved shape to the vertical plate's
contour line, similar to that shown in FIG. 3c, instead of a straight
shape. As schematically shown in this Figure, a corrugated shape is given
to this plate, performing a number of vertical lobes (31) on it. By doing
so, the conduction path through the metal is constrained, and the heat
flux crossing this path is significantly reduced.

[0100] Many of the features described here are implemented for different
embodiments of the barrier. Nevertheless, many combinations of them can
be implemented for a single barrier. For example, the waved shape of the
barrier outer shell near its outer perimeter as well as the non-planar
geometry for any or both of the upper and lower plates (21a, 21b) of the
barrier shell, can be added at the same time to the barrier.